1. Field of the Invention
The present invention relates to a temperature-compensated crystal oscillator for use in an electronic device such as a mobile communication device.
2. Description of the Related Art
Conventionally, temperature-compensated crystal oscillators are employed in electronic devices such as mobile communication devices.
A conventionally known temperature-compensated crystal oscillator has a construction such that a container 23 is bonded onto an upper surface of a planar substrate 21 having a plurality of external terminals 22 provided on a lower surface thereof as shown in FIGS. 21(a) and 21(b). A crystal oscillator element 24 is accommodated in the container 23. An IC element 26 which controls an oscillation output on the basis of the oscillation of the crystal oscillator element 24 is provided in a cavity 25 defined by a lower surface of the container 23 and an interior surface of a frame base 27 attached to the lower surface of the container 23.
The container 23 is adapted to hermetically seal the crystal oscillator element 24 accommodated therein without communication with the atmosphere. The container 23 includes a base plate composed of an electrically insulative material and a seal ring 31 attached to an upper surface of the base plate. The crystal oscillator element 24 is attached to the upper surface of the base plate in the seal ring 31. A metal lid 32 is welded to an upper surface of the seal ring 31 by seam welding(resistance welding), whereby the space in which the crystal oscillator element 24 is accommodated is hermetically sealed.
In general, the base plate of the container 23 and the frame base 21 are integrally formed of a ceramic material such as glass-ceramic. Interconnection conductors are provided in and on the base plate and the frame base. A conventionally known ceramic green sheet laminating method is employed for the formation of the base plate and the frame base.
An IC element 26 mounting area on the planar substrate 21 is illustrated as having a rectangular shape by a solid line in FIG. 22. As shown, a plurality of electrode pads 28 respectively connected to electrodes of the IC element 26 are arranged in two rows as stated, for example, in Japanese Unexamined Patent Publication No. 2001-291742. Five electrode pads are arranged in one of the two rows, and six electrode pads are arranged in the other row.
A flip chip IC element 26 having a plurality of connection pads is employed as the IC element 26. Where the flip chip IC element 26 is mounted on a lower side of a mounting base 23, the connection pads of the IC element 26 are first brought into contact with the corresponding electrode pads 28 in the IC element 26 mounting area with the intervention of electrically conductive bonding members such as of a solder, and then the electrically conductive bonding members of the solder are heat-melted at a high temperature for the mounting of the IC element 26 on the mounting base 23.
However, the plan area of the aforesaid temperature-compensated crystal oscillator is reduced to 7 mm×5 mm, to 5 mm×3 mm, and further to 3 mm×2 mm. With the size reduction of the oscillator, there is an increasing demand for the size reduction of the IC element 26. As a result, the pitch of the electrode pads 28 should be reduced with the need for the reduction of the IC element 26 mounting area. This reduces the bonding reliability, leading to limited flexibility in routing interconnection conductors 29 connected to the electrode pads 28.
In order to flatten the oscillation output by oscillation control by means of the IC element 26 according to the specific temperature characteristic of the crystal oscillator element 24 of the temperature-compensated crystal oscillator, the specific temperature characteristic of the crystal oscillator element 24 should preliminarily be measured before the mounting of the IC element 26. However, the size reduction of the electrode pads makes it difficult to measure the specific temperature characteristic, drastically reducing the productivity.
An advantage of the present invention is to provide a temperature-compensated crystal oscillator which totally has a reduced size and yet maintains and improves the bonding reliability of an IC element.